The present invention was developed by the present inventors as part of a project by the Engineering Clinic of Harvey Mudd College for Rancho Los Amigos Hospital for aid in the treatment of patients with abnormal gait due to lower extremity malfunction. The analysis of foot forces helps in locating the cause and severity of the disability and to help in evaluating past treatment, either surgical or therapeutic, as well as providing information for future treatment.
The most commonly used system in the prior art has been a force-sensitive plate imbedded in a walking surface. However, such plates typically are suitable only for measuring the total forces over the entire foot occurring during a single step, and require the patient to walk over the plate several times to obtain a sufficient data base. Such a time consuming and repetitive procedure may be so painful to some patients as to be intolerable. Further, while such plates enable accurate data to be obtained, since they typically employ piezoletric sensors and charge amplifiers, the sensors tend both to limit plate size and make cost prohibitive. Additionally, since such plates sense only total foot forces, it is impossible to determine the areas of the foot where the forces are being exerted.
Photographic techniques have also been used in documenting defects in gait, but such techniques tend to involve considerable expense. A third method, that of placing microswitches in various positions in the insole of the patient's shoe, permits forces at any position on the foot to be sensed, but gives only a position indication and not an indication of the magnitude of the forces.
An earlier, less successful transducer developed in 1974 by others at the Engineering Clinic of Harvey Mudd College employed a configuration similar to the present invention, although different in several important respects. The earlier configuration used a slotted transducer plate as with the present invention, a single strain gauge and two Belleville springs, rather than a single spring and two strain gauges as in the present invention. The single strain gauge was positioned on the transducer plate between the two slots and perpendicular thereto and, similar to that described hereinafter for the present invention, the two Belleville springs were fitted into a retaining lip on either side of the transducer plate. Thus a force applied to the transducer created primarily an axial strain in the transducer plate since any bending moment was minimized by the use of a Belleville spring on either side of the transducer plate. In constrast and as will be described in detail hereinafter, the present invention operates by monitoring the bending moment in the transducer plate. In addition, the previous transducer suffered from poor linearity of response and exhibited rather high hysteresis, whereas the present invention maintains good linearity and substantially reduced hysteresis.
Thus the need has existed for an accurate transducer small enough to permit placing several such transducers on a single shoe, but which indicates the magnitude of a force, without significantly interfering with the patient's gait.